You can run an 18 inch tube on either, but it won't be running the velocities to get the full benefit of the 6.5 pills. Coolness doesn't translate to hits.
I wouldnt go to short on the 6.5 build stay around 24 or 26. ive had good luck with both.
But on the 260 you can run a tight bore and go as short as 18. thats what Ive got on one of my 260's and the other one if a Badger build on a 24. they are both awesome but I prefer the 18 barrel.